1. Field of the Invention
The present invention relates in general to methods and apparatus for manufacturing piston rods, and, more particularly, to a method for welding a ball to the end surface of a rod through friction or projection, as well as to apparatus useful for effecting such method.
2. Description of the Prior Art
Generally, a piston rod with a ball welded to both ends of the rod through friction is accomplished by use of a variable capacity compressor as disclosed, for example, in Japanese Patent Publication No. 23390/1986.
Prior art piston rods of such type are disclosed in Japanese Patent Laid-Open No. 124815/1983 and manufacturing methods therefor are shown in Japanese Patent Laid-Open No. 54346/1973 and Japanese Patent Laid-Open No. 128522/1983.
FIGS. 17 to 26, inclusive of the drawing of this application are presented to reveal graphically prior art pistons of such type, whereby a more comprehensive view may be obtained of the background for the present invention with the attributes thereof becoming all the more apparent. Thus, with reference to FIG. 17, the piston rod depicted comprises a rod 1 constructed of, for example, S20CD steel, having welded at each of its ends, through friction, a ball 2, as of SUJ2 steel.
FIG. 18 schematically illustrates a conventional method for producing the piston rod, shown in FIG. 17., wherein each ball 2 and the rod 1 are subjected to friction welding by butting the rod 1 against each ball 2 as the rod is rotated at high speed. However, by friction welding of each ball 2 to the rod 1 is performed under atmospheric conditions an oxide 3 develops on the surface of the butt weld zone of rod 1 and each ball 2, which extends lengthwise a distance, for example of 5 to 9 mm (See FIG. 19). Understandably, the formation of such oxide 3 causes a deterioration of the quality of the piston rod, with loss of dimensional accuracy as may be required design-wise. Accordingly, a post-welding process, such as grinding or the like, is required, all of which results in an increase in manufacturing cost.
FIG. 20 illustrates a conventional apparatus for manufacturing the piston rod shown in FIG. 17, and therein reference numeral 9 denotes a holder for mechanically retaining ball 2. Holder 9 is shiftable axially by operation of a cylinder 10. A holder 11 is provided for presenting a rod 6 against a ball 7. Rod holder 11 is connected to a rotor 14 which driven by a motor 13 through a clutch 12. A brake disk 16 for engagement with a brake 15 is fixed on rod holder 11.
The operation of the apparatus shown in FIG. 20 is as follows: ball 7 having a formed end surface 8, as shown in FIG. 21, as by cutting, is held mechanically on ball holder 9; the rod 6 is suitably held on rod holder 11 and motor 13 is then energized to rotate rod holder 11. Cylinder 10 is actuated to move ball holder 9 in order to present ball end surface 8 to the adjacent end surface of rod 6 whereby welding rod 6 to ball 7 through friction is effected, with the resulting structure illustrated in FIG. 22. However, according to such conventional method, since ball 7 is held on holder 9 through mechanical clamping, set up time is required for appropriate centering, thus decreasing working efficiency.
On the other hand, ball 7 and rod 6 must be held by a considerably strong force for friction welding, so that the clamp mechanism for ball holder 9 and rod holder 11 is relatively complicated.
FIG. 23 shows a piston rod for the described type which is formed through projection welding, wherein a piston rod 4, is of S20CD steel, is formed of a ball 5, for example, of SUJ2 steel, welded through projection, to each end of said rod 4 which is retained on another electrode. After bringing an end surface of rod 4 into contact with a ball 5, a carrier is developed between the opposed electrodes, thereby welding rod 4 to balls 5. However, by projection welding, the projection weld zone is hardened by intense heat, wherefore a hard, yet fragile structure is formed on the connection between the rod and each ball, with an unusual deterioration in properties. In piston rods for which a predetermined mechanical strength is required, the projection weld zone must be tempered after welding. For tempering, the welded piston rod is transferred to a heat treatment apparatus which thus necessitates a heat treatment process in addition and hence causes a diminution in production efficiency.
Furthermore, such conventional method is customarily capable of causing a sizeable discrepancy in overall length (dimension L in FIG. 23) of the piston rod as thus welded by projection, so that quality control is indeed extremely difficult.
As shown in FIG. 24, in case balls 5 of a predetermined diameter are subjected to projection welding to the ends of the 200 rods 4 substantially of identified length, the overall length L of the piston rod is distributed over a substantial range, as graphically shown in FIG. 25, as with 35 pieces between 58.2 mm and 58.4 mm; 34 pieces between 58.4 mm and 58.8 mm and 87 pieces between 58.8 mm and 59.0 mm. The then length of penetration is also distributed over an extensive range, as illustrated in FIG. 26; and with 45 pieces between 1.4 mm and 1.6 mm; 75 pieces between 1.6 mm and 1.8 mm; 29 pieces between 1.8 mm and 2.0 mm; 30 pieces between 2.0 mm and 2.2 mm; 21 pieces between 2.2 mm and 2.4 mm.
Consequently, there has been hitherto the constant demand that piston rod manufacturing apparatus be provided which is capable of assuring the overall length L of a piston rod, after projection welding of high dimensional accuracy.